Custom Fields, The Loop and Last Modified

Hello everyone, just a general query really with regards to editing the post loop, so that posts are displayed in last-modified order BUT only is a certain criteria is met in a particular custom field.

To put this in context – I have a discount and voucher codes website which lists manually updated posts containing discount codes in last modified order but sometimes these codes expire – so I don’t want them displayed when they have expired. So I’d have a custom field that would set a flag to not display in the loop (for the homepage) but would still display in the relevant category.

I hope this makes sense?!!?? Can anyone help please?

Thanks in advance ??